Diesel vehicles are equipped with an aftertreatment system which may include, for example, one or more of each of a selective catalytic reduction (SCR) system and a diesel particulate filter in order to reduce emissions. Such an aftertreatment system may utilize the injection of a reductant such as urea to facilitate the reduction of NOx, for example. An injection amount of urea that is too low may result in a NOx conversion efficiency that is too low to meet regulation standards. On the other hand, an injection amount of urea that is too high may result in urea deposits in the system which may also decrease NOx efficiency and increase urea slip, as well as generate increased white smoke in the exhaust at high temperatures when the deposit is decomposed and released. Further, injection of too much urea may increase urea consumption thereby reducing urea economy.
The inventors herein have recognized the above issues and have devised an approach to at least partially address them. Thus, a method for an aftertreatment system of an engine exhaust, the aftertreatment system including a SCR catalyst and a particulate filter (PF), is disclosed. The method comprises, during decreased soot production, decreasing a total amount of urea injected to the SCR catalyst between a first and second regeneration and, during increased soot production, increasing the total amount of urea injected to the SCR catalyst between the first and second regeneration.
In one example the amount of urea injection is limited below a threshold, the threshold based on engine soot generation in addition to parameters such as exhaust temperatures, exhaust flow rates, and NO emission from the engine. For example, a urea deposit removal rate may be related to that of a burn rate of soot in a PF; thus, it may be beneficial to adjust urea injection to generate a urea deposit formation rate that is related to (e.g., less than) a soot deposit rate in order for a PF regeneration to substantially remove accumulated urea deposits. In other words, as PF regenerations may be triggered by soot storage levels (which in turn are driven by soot deposit rates and thus soot generation rates), if the urea deposit rate is controlled (through limiting urea injection levels) based on the soot levels, the PF regenerations triggered based on soot will be often enough to remove any urea deposits.
As such, during decreased soot production when the average engine out soot amount per unit time is decreased, the amount of urea injected to the SCR catalyst may limited to a reduced level, and the total amount of urea injected during decreased soot production is less than that during an interval of increased soot production when the average engine out soot amount per unit time is increased. In this way, an amount of urea injected to the SCR catalyst may be controlled such that urea economy and/or NOx conversion efficiency may be increased and accumulated soot deposits in the SCR catalyst may be reduced resulting in a reduced amount of white smoke in the exhaust, for example.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.